Thick bed fiber filters, also called candles, aerosol filters, mist filters and mist eliminators represent the most advanced method of collection of fine particulates having a diameter of less than three microns. This type of device has an efficiency in excess of 99% for particles having a diameter as small as 1/10th of a micron. Processed gas contaminants, such as fumes, smog, oil and resin smoke, are typically one micron or less in diameter.
These units are widely used in many industries to remove liquid and semi-liquid mists carried in a process gas. In many cases, solid particulates are also present as a result of the process operations. Unfortunately, the candles are as efficient for solids as they are for liquids. Because liquids drain continuously they do not present a problem. Solids, however, tend to remain in the filter, trapped between the interstices of the filter media. Eventually, the filter becomes plugged rendering it ineffectual. As the filter becomes impregnated with solids, the pressure drop across the filter increases over a period of time but efficiency remains constant. A filter change must be made to return the pressure drop to a satisfactory level.
Prior art techniques for ameliorating this problem utilize "socks" to protect the candles from plugging by accumulation of solid particles. The socks comprise a layer of filter media having an average packing density of only 2 pounds per cubic foot. The sock is wrapped around the outside of the candle to trap particles before they reach the aerosol primary filter. It was thought that this would extend the useful life of the primary filter. This concept did not work very well because of the low packing density and large diameter fibers of the sock material. Other methods of pressure drop recovery have included washing in place or washing externally using the appropiate washing medium to dissolve the solids and semi-solids from the filter media or by installing a new set of filters. Washing is expensive and time consuming. Moreover, the degree of recovery achieved using such a procedure is affected by the type and size of collected solids and the rapidity of solid dissolution by the washing media. Experience has shown that only a portion of the pressure drop is recovered and the effect of the wash cycle will eventually require main filter replacement.